This invention relates to weather radar systems for obtaining rain information by radiating microwave pulses from a radar transmitter and receiving echo signals from weather targets at a radar receiver, and, in particular, to rejection of echo signals from fixed targets such as the ground (which are hereinafter referred to as "ground clutter") included in the received signals.
In weather radar system, microwave pulses are radiated with a certain repetition frequency from the transmitter and echo signals from weather targets are received at the receiver. The received wave is envelope-detected to provide an echo signal corresponding to each pulse radiation.
The reflection intensity of particles of the weather target within an echoing volume varies and, therefore, the amplitude of the echo signal fluctuates rapidly from one pulse to the next. But since the particles, or rain drops, are randomly distributed within the echoing volume, the average echo intensity over a number of pulses is the sum of echo intensities of individual particles.
Therefore, physical properties of the weather such as rain amount can be detected by measuring the average echo intensity, or mean power of the weather echo signals over a number of pulses.
The echo signals which are successively provided accordingly to radiated pulses, are averaged and applied to an indicator to display the geometric pattern and motion of weather targets such as rain, clouds, storms and so on.
As indicators there are known a plan position indicator (PPI), an A-scope indicator and others.
As well known in the prior art, since the average intensity of a weather echo depends on, for example the rain amount, the amplitude of the averaged signal in the radar receiver enables one to know rain amount. Therefore, for example, the rain amount at a certain distance from the radar can be seen from the brightness of the pattern on the PPI scope. Furthermore, an iso-echo indicator is used to display an iso-rain pattern by slicing the averaged signals at a predetermined amplitude level.
Moreover, the echo signals or the averaged signals are inputted into a computer or a data processing device to obtain various weather information.
One of the difficult problems in weather radar systems is rejection of ground clutter. The reflected wave received at the radar receiver includes echoes from fixed objects such as mountains, buildings and other objects on the ground (this echo is referred to as "ground clutter"), besides weather echos. The ground clutter intrudes in echo signals of the weather target to make it difficult to obtain the weather pattern and other information from the echo signals.
To eliminate the ground clutter, the radar observer or the meteorologist must draw the weather pattern viewing the pattern on the PPI scope, and use his own experience and knowledge.
One of the known methods for eliminating ground clutter is to use a memory storing ground echoes at fine weather, and to subtract the stored ground clutter signal from echoes observed at rain. In this method, a large capacity memory device is required to store ground clutter at various directions and ranges. For example, to store a ground clutter pattern within 300 Km around the radar device, a memory capacity of 108,000 words is required when the sampling period in the distance and the bearing are 1 Km and 1.degree., respectively, with a wave, or antenna, elevation angle being fixed. In this method, since the stored ground echoes must be subtracted from the observed echoes, an expensive computer is required for the operation and real time processing is impossible.
Furthermore, the effective elimination of ground clutter cannot be achieved, because the intensity of ground echoes considerably varies for several hours dependent on the change of the electric wave propagation condition, and of the water content of the ground, woods, buildings and others.
Another known method for removing the effects of ground clutter is to abandon the echo signals including ground clutter by detecting the existence of ground clutter. The detection is achieved by observing an unchanging component within successive echo signals on the basis that the amplitude of ground clutter does not change for a short time period such as the radiated pulse to pulse interval. But in this method almost all data obtained by the radar 15 abandoned in a certain area so that the weather observation by the use of the radar is impossible in the area.